NEW YORK (AdAge.com) -- Imagine a world where only half of the visitors to FoxSports.com had watched a sporting event in the past week, or where students comprised less than 5% of visitors to MTV.com.

That's the new world publishers are facing as a result of changes the Nielsen Co. is making to its @Plan service, one of the most oft-used systems agencies use to slice and dice online audiences by age, demographics and psychographics -- and target them with some of the online world's $7.6 billion in U.S. advertising dollars, a figure that excludes search.

In October, Nielsen told clients that it was going to change its methodology for @Plan, including more than tripling the number of sites included in the data, and shifting from a questionnaire to electronic meters to determine who had visited which sites. It also lowered the minimum size of a sample on which it collects data to 50 from 200.

Nielsen still uses quarterly surveys to determine offline behavior, and this is combined with electronic measurement to come up with the new data. Theoretically, the changes would produce much more granular results and perhaps remove an inherent bias toward well-known brands.

The new system is still in beta, and Nielsen is giving publishers time to work with it before it becomes de facto currency in February. But some big sites are getting a shock and seeing wide swings from what the old data said about their audiences.

Lower numbers

Multiple publishers passed along data they consider suspect from the @Plan system. Among them: The old data said 76% of FoxSports.com audiences had watched a game on TV in the previous week. The new numbers say just 50%. The old @Plan data had 11.1% of MTV.com visitors identified as students; the new data say just 4.5%.

It gets more striking: Under the old system, 32% of People.com visitors also read the magazine. The new data say just 8% had read the magazine the prior week.

Other sites experiencing big swings in some of their core numbers include Johnson & Johnson's Babycenter, CBS's Gamespot, NBC's iVillage, Time Warner's TMZ.com, Disney's ESPN.com and Oprah.com, according to an exec who analyzed the data, but he added that you don't have to look too deep to see discrepancies.

There are other services that link online demographics to offline activities, but none as widely used or trusted by planners as Nielsen's @Plan. When agency buyers are putting together a media plan, they'll check @Plan for the top sites reaching their target audience -- and then buy those sites.

Nielsen declined to comment on the new @Plan results except to say that the changes are still in beta and the company is consulting with clients to work out whatever kinks may exist. Several executives at publishers contacted by Ad Age said they had meetings scheduled with Nielsen to go over the results.

Explanations

One observer chalked it up to the fact that when you mix surveys with electronic collection, strange things can happen to the data. It could also be true that the electronic measurement is revealing the small sites are getting a lot more traffic that previously thought.

Still, one said the numbers shouldn't change substantially from the system the industry has been using for the past decade. "We assumed the data would be more or less directionally accurate," the exec said. "But the data is moving in the opposite direction."

Initially, Nielsen had hoped to have the new system in place by November, but that's been pushed out to February, allowing for more time to listen to clients and to get the system right.

It's a sensitive issue for publishers, already under pressure in a tight ad market and desperate to set themselves apart in front of marketers.

Andrea Kerr Redniss, managing director at Publicis' Optimedia, said agencies probably won't notice the change, unless it's severe. "On the agency side if they are still in the top five we wouldn't really take notice," she said. "Sites are all claiming to be No. 1 if you cut the data some way, so if they fell a bit we would have just assumed we pulled the data differently."

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